Fixes notice in DisjointIterator, a=chris

diff --git a/src/library/index_bundle_iterators/DisjointIterator.php b/src/library/index_bundle_iterators/DisjointIterator.php
index 682e7526b..60bd88d41 100644
--- a/src/library/index_bundle_iterators/DisjointIterator.php
+++ b/src/library/index_bundle_iterators/DisjointIterator.php
@@ -140,7 +140,10 @@ class DisjointIterator extends IndexBundleIterator
         //next we finish computing BM25F
         $docs = $this->index_bundle_iterators[
             $this->least_offset_index]->currentDocsWithWord();
-        $this->count_block = count($docs);
+        $this->count_block = 0;
+        if (is_array($docs)) {
+            $this->count_block = count($docs);
+        }
         $this->pages = $docs;
         return $docs;
     }
@@ -256,4 +259,3 @@ class DisjointIterator extends IndexBundleIterator
         }
      }
 }
-